Methods for preparing mixed-metal oxide diamondoid nanocomposites and catalytic systems including the nanocomposites

ABSTRACT

Methods for preparing a layered metal nanocomposite and a layered metal nanocomposite. The method includes mixing a magnesium salt and a aluminum salt to form a Mg 2+ /Al 3+  solution. The Mg/Al has a molar ratio of between 0.5:1 to 6:1. Then a diamondoid compound is added to the Mg 2+ /Al 3+  solution to form a reactant mixture. The diamondoid compound has at least one carboxylic acid moiety. The reactant mixture is heated at a reaction temperature for a reaction time to form a Mg/Al-diamondoid intercalated layered double hydroxide. The Mg/Al-diamondoid intercalated layered double hydroxide is thermally decomposed under a reducing atmosphere for a decomposition time at a decomposition temperature to form the layered metal nanocomposite.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/508,672 filed May 19, 2017, which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present specification generally relates to preparation ofnanocomposites and to catalytic systems including the nanocompositesand, more specifically, to synthesis of mixed metal-oxide diamondoidnanocomposites and catalytic systems containing the nanocomposites.

BACKGROUND

Catalyst materials may be produced from anionic clays such as layereddouble hydroxides (LDHs). Anionic clays are inverse charge analogs ofthe widely used aluminosilicate cationic clays in their structure andproperties. The largest group of the LDH family of materials includespositively charged metal hydroxide layers having the composition [M^(II)_(1-x)M^(III) _(x)(OH)₂]^(x+) (M^(II)=Mg, Ca, Co, Ni, Zn; M^(III)=Al,Cr, Fe; 0.2≤x≤0.33). The positive charges on the hydroxide layers arebalanced by anions between the layers. The anions give rise to the nameanionic clays. One group of anionic clays includes materials having ageneral formula [M^(III) _(1-x)M^(III) _(x)(OH)₂](A^(n−))_(x/n).mH₂O(m=0.33-0.50), where A is an anion such as nitrate or halogen.

LDHs are environmentally benign and economically viable layeredmaterials. Owing to their readily varied composition, well-dispersedsubstitutions, and layered morphology, these materials have found use invarious applications. Thermal decomposition of LDHs results inmixed-metal oxides that are chemically basic. These mixed-metal oxideshave potential for use as heterogeneous catalysts in various catalyzedreactions, including the water gas shift reaction and photocatalyticapplications. In addition, these mixed-metal oxides may be suitable forcapturing CO₂ from coal fired power plants that emit large amounts ofCO₂ into environment. In one or more applications, mixed-metal oxidematerials obtained from LDHs have been found to be suitable sorbents forcapturing acidic CO₂ gas and to be capable of adsorbing toxic ions fromindustrial effluents and drinking water.

The synthesis of supported metal or metal-oxide catalysts is importantto the field of industrial heterogeneous catalysts. High activity, highselectivity, and long catalyst life are desirable characteristics of anyindustrial catalyst.

SUMMARY

Ongoing needs exist for methods to synthesize active and selectivecatalysts that are also environmentally friendly. Accordingly,embodiments of this disclosure include methods for synthesizingnanocomposites including metal oxide particles obtained by decomposingLDHs. In particular, the synthesis methods may be conducted as a “onepot” synthesis, without a need for multiple washing steps.Nanocomposites prepared according to the methods may be incorporatedinto catalytic systems.

According to some embodiments, a method for preparing a layered metalnanocomposite includes mixing a magnesium salt and an aluminum salt toform a Mg²⁺/Al³⁺ solution. The Mg²⁺/Al³⁺ solution has a molar ratio ofMg:Al of between 0.5:1 to 6:1. Subsequently, a diamondoid compound isadded to the Mg²⁺/Al³⁺ solution to form a reactant mixture. Thediamondoid compound has at least one carboxylic acid moiety. Thereactant mixture is heated at a reaction temperature for a reaction timeto form a Mg/Al-diamondoid intercalated layered double hydroxide. TheMg/Al-diamondoid intercalated layered double hydroxide is thermallydecomposed under a reducing atmosphere for a decomposition time at adecomposition temperature to form the layered metal nanocomposite.

Some embodiments include layered metal nanocomposites prepared accordingto the method of this disclosure. The layered metal nanocompositesinclude magnesium oxide (MgO) comprising 50 wt. % to 90 wt. % of thenanocomposite based on the total weight of the nanocomposite. The MgOmay include particle sizes of from 10 nanometers (nm) to 20 nm.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a Powder X-Ray Diffraction (PXRD) pattern of aMg/Al-adamantoate LDH according to one embodiment.

FIG. 2 is an Infrared (IR) transmittance spectrum of a Mg/Al-adamantoateLDH according to one embodiment.

FIG. 3 is a proton solid-state Nuclear Magnetic Resonance (NMR) spectrumof a Mg/Al-adamantoate LDH according to one embodiment.

FIG. 4 is a carbon-13 (¹³C) solid-state NMR spectrum of aMg/Al-adamantoate LDH according to one embodiment.

FIGS. 5A and 5B are Scanning Electron Microscopy (SEM) micrographs atdifferent magnifications of an Mg/Al-adamantoate LDH prepared accordingto the present disclosure.

FIG. 6 is a PXRD pattern of a nanocomposite formed from Mg/Al-adamantaneintercalated LDH and decomposed at 450° C. in a hydrogen atmosphere inaccordance with one or more embodiments of the present disclosure.

FIG. 7 is an IR spectrum of a nanocomposite formed from Mg/Al-adamantaneintercalated LDH and decomposed at 450° C. in a hydrogen atmosphere inaccordance with one or more embodiments of the present disclosure.

FIG. 8 is a ¹³C NMR of a nanocomposite formed from Mg/Al-adamantaneintercalated LDH and decomposed at 450° C. in a hydrogen atmosphere inaccordance with one or more embodiments of the present disclosure.

FIG. 9 is a ²⁷Al NMR of a nanocomposite formed from Mg/Al-adamantaneintercalated LDH and decomposed at 450° C. in a hydrogen atmosphere inaccordance with one or more embodiments of the present disclosure.

FIG. 10A-10D are SEM micrographs of a nanocomposite formed fromMg/Al-adamantane intercalated LDH and decomposed at 450° C. in ahydrogen atmosphere in accordance with one or more embodiments of thepresent disclosure.

FIGS. 11A-11D are SEM micrographs of a nanocomposite formed fromMg/Al-adamantane intercalated LDH and decomposed at 450° C. in ahydrogen atmosphere in accordance with one or more embodiments of thepresent disclosure.

FIGS. 12A-12C are transition electron microscopy (TEM) micrographs of ananocomposite formed from Mg/Al-adamantane intercalated LDH anddecomposed at 450° C. in a hydrogen atmosphere in accordance with one ormore embodiments of the present disclosure.

FIG. 12D is a high resolution transition electron microscopy (HRTEM)micrograph of a nanocomposite formed from Mg/Al-adamantane intercalatedLDH and decomposed at 450° C. in a hydrogen atmosphere in accordancewith one or more embodiments of the present disclosure.

FIGS. 13A-13C are TEM micrographs of a nanocomposite formed fromMg/Al-adamantane intercalated LDH and decomposed at 450° C. in ahydrogen atmosphere in accordance with one or more embodiments of thepresent disclosure.

FIG. 13D is a HRTEM micrograph of a nanocomposite formed fromMg/Al-adamantane intercalated LDH and decomposed at 450° C. in ahydrogen atmosphere in accordance with one or more embodiments of thepresent disclosure.

FIG. 14 is an energy dispersive X-ray (EDX) spectrum of a nanocompositeformed from Mg/Al-adamantane intercalated LDH and decomposed at 450° C.in a hydrogen atmosphere in accordance with one or more embodiments ofthe present disclosure.

FIG. 15A is an STEM micrograph of a nanocomposite formed fromMg/Al-adamantane intercalated LDH and decomposed at 450° C. in ahydrogen atmosphere in accordance with one or more embodiments of thepresent disclosure.

FIG. 15B is an elemental mapping of magnesium in the STEM micrograph ofFIG. 15A.

FIG. 15C is an elemental mapping of oxygen in the STEM micrograph ofFIG. 15A.

FIG. 15D is an elemental mapping of carbon in the STEM micrograph ofFIG. 15A.

FIG. 15E is an elemental mapping of aluminum in the STEM micrograph ofFIG. 15A.

DETAILED DESCRIPTION

Abbreviations appearing in this disclosure are defined as follows: °C.=Degrees Celsius; A=Angstroms; ACA=1-adamantane carboxylic acid;AD=adamantane; cm=centimeter (10⁻² meter); EDX=Energy-Dispersive X-Ray;FWHM=full width at half maximum; h=hours; HRTEM=High-ResolutionTransmission Electron Microscopy; IR=Infrared; LDH=layered doublehydroxide; μm=micrometer (10⁻⁶ meter); mL=milliliter (10⁻³ liter);nm=nanometer (10⁻⁹ meter); ppm=parts per million; PXRD=Powder X-RayDiffraction; SEM=Scanning Electron Microscopy; TEM=Transmission ElectronMicroscopy; TGA=Thermogravimetric Analysis; TMO=Transition metal oxide;and wt. %=Weight percent.

The dispersion of active reduced-metal or metal-oxide particles on astable support is a complex and labor-intensive process requiringconsideration of multiple parameters such as synthesis conditions,properties of the support, and appropriate methods for dispersing ordistributing active catalyst on the support. Among metal/metal oxidesupported catalysts, Cu/ZnO/Al₂O₃ systems and metal/metal oxide (Pt, Pd,Rh and Au) systems supported on various supports (alumina, silica, andcarbon) can catalyze industrial-scale reactions such as synthesis ofmethanol, water gas shift reaction, desulfurization of petrochemicalstreams, photochemical or electrochemical splitting of water, andphotochemical or electrochemical reduction of carbon dioxide into usefulchemicals, for example.

Reference will now be made in detail to embodiments ofadamantane-intercalated layered double hydroxide (LDH) particles withhigh aspect ratios and the methods of producing them. Specifically, theadamantane-intercalated LDH particles have aspect ratios greater than100. The aspect ratio is defined by the width of the LDH particledivided by the thickness of the LDH particle. As used in thisdisclosure, the term “low aspect ratio” refers to an aspect ratio lessthan 10; a “medium aspect ratio” is an aspect ratio from 10 to 100; anda “high aspect ratio” is an aspect ratio greater than 100. Aspect ratiosof individual LDH particles may be calculated from micrographs such asSEM images.

Methods for preparing diamondoid-intercalated LDH particles,specifically adamantane-intercalated LDH particles, are described incommonly assigned United States Application Publication Number2017/0267623, published Sep. 21, 2017, claiming the benefit of U.S.Provisional Application Ser. No. 62/309,645, filed Mar. 17, 2016, bothincorporated by reference into this disclosure in their entirety. UnitedStates Application Publication Number 2017/0267623 describes methods forpreparing diamondoid-intercalated mixed-metal LDH. Thesediamondoid-intercalated mixed-metal LDH were decomposed tonanocomposites under air. The presence of oxygen alters thedecomposition process and the resulting nanocomposite is structurallyand chemically different from a nanocomposite formed by decomposing adiamondoid-intercalated mixed-metal LDH in a reducing atmosphere.

As used in this specification, the term “diamondoid” refers to variantsof carbon cage molecules known as adamantane (C₁₀H₁₆). The carbon cagesinclude tri-, tetra-, penta, and polycyclic structures. In someembodiments, diamondoid includes adamantane, diamantane, triamantane andhigher polymantanes. The diamondoid compounds may include a functionalgroup such as carboxylic acid, hydroxyl, carboxylic ester, or amine. Insome embodiments, the diamondoid compound is 1-adamantane carboxylicacid.

The methods for preparing a layered metal nanocomposite include mixing amagnesium salt and an aluminum salt to form a Mg²⁺/Al³⁺ solution, inwhich the molar ratio of magnesium to aluminum is from 1:1 to 6:1. Adiamondoid compound is added to the Mg²⁺/Al³⁺ solution to form areactant mixture. The diamondoid compound has at least one carboxylicacid moiety. The reaction mixture then is thermally treated at areaction temperature for a reaction time to form adiamondoid-intercalated Mg/Al LDH. The diamondoid-intercalated Mg/Al LDHis thermally treated in a reducing atmosphere for a decomposition timeat a decomposition temperature. The thermal treatment in the reducingatmosphere decomposes the diamondoid-intercalated Mg/Al LDH to form thelayered metal nanocomposite.

In some embodiments, the Mg²⁺/Al³⁺ solution is an aqueous solution. Anaqueous solution can be any suitable fluid such as water or a solutioncontaining both water and one or more organic or inorganic compoundsdissolved in the water or otherwise completely miscible with the water.

In one or more embodiments, the magnesium salt of the Mg²⁺/Al³⁺ solutionmay include any magnesium compound containing Mg²⁺ and a counter anion.Non-limiting examples of magnesium salts, therefore, include Mg(OH)₂,MgCl₂, MgBr₂, Mg(NO₃)₂, and MgSO₄. In some embodiments, the magnesiumsalt may be Mg(OH)₂. MgO formed from the calcination of Mg(OH)₂ is ofparticular interest for its activity as a solid base catalyst.

In some embodiments, the aluminum salt in the Mg²⁺/Al³⁺ solution mayinclude any aluminum compound containing Al³⁺ and a counter anion.Non-limiting examples of aluminum salts include any soluble aluminumsalt, such as Al(OH)₃, AlCl₃, AlBr₃, AlI₃, Al₂(SO₄)₃, Al(NO₃)₃, andAlPO₃. Al₂O₃ formed from the calcination of Al(OH)₃ is of particularinterest for its activity as a solid base catalyst.

In the reactant mixture, the diamondoid compound has at least onecarboxylic acid moiety. In some embodiments, the diamondoid compound maybe chosen from carboxylic acids of adamantane, diamantane, ortriamantane. In some embodiments, the diamondoid compound may be1-adamantanecarboxylic acid (ACA).

In some embodiments of methods for preparing a layered metalnanocomposite, the reaction mixture may be prepared by mixing amagnesium salt such as, for example, Mg(OH)₂, and the aluminum salt suchas Al(OH)₃ in amounts that provide a molar ratio of Mg²⁺ to Al³⁺ in thereaction mixture of from 0.5:1 to 6:1. For example, the Mg²⁺/Al³⁺solution may have a Mg²⁺/Al³⁺ molar ratio of 1:1, 2:1, 3:1, 4:1, 5:1, or6:1. The Mg²⁺/Al³⁺ solution may have a total solids content of less than15 wt. % based on a total weight of the Mg²⁺/Al³⁺ solution. The totalsolids content may include any solid compound added to the Mg²⁺/Al³⁺solution. Specific examples of solid compounds added to the Mg²⁺/Al³⁺solution that are counted as part of the total solids content include,but are not limited to, the magnesium salt, the aluminum salt, and thediamondoid compound. In some embodiments, the total solids content islimited to include the magnesium salt, aluminum salt, and the diamondoidcompound. In some embodiments, the total solid content of the Mg²⁺/Al³⁺solution is from 0.1 wt. % to 15 wt. %, 0.5 wt. % to 10 wt. %, or isless than 5 wt. %.

In one or more embodiments, the methods for preparing a layered metalnanocomposite may include adding an amount of diamondoid compound to theMg²⁺/Al³⁺ solution to form a reaction mixture having an Al to diamondoidcompound molar ratio of from 0.5:1 to 2:1. In some embodiments, the Alto diamondoid compound molar ratio in the reaction mixture may be from0.8:1.0 to 1.2:1.0. For example, the molar ratio of Al to the diamondoidcompound may be 1:1.

The specific molar ratio of Mg²⁺ to Al³⁺ and Al³⁺ to diamondoid compoundin the reaction mixture may be chosen to tailor overall crystalmorphology of the diamondoid-intercalated Mg/Al LDH. Without intent tobe bound by theory, it is believed that the crystal morphology of thediamondoid-intercalated Mg/Al LDH may be tailored by increasing ordecreasing the ratio of Al³⁺ to ACA in the reaction mixture. Though insome embodiments the ratio of Al³⁺ to diamondoid compound may beselected from 0.5:1 to 1.0:1, it should be understood that the crystalmorphology of the diamondoid-intercalated Mg/Al LDH may be furthertailored by decreasing the ratio of Mg²⁺ to diamondoid compound to lessthan 0.5:1 or by increasing the ratio of Mg²⁺ to diamondoid compound togreater than 1.0:1. Even so, a point of magnesium saturation is believedto exist, such that at a ratio of Mg²⁺ to diamondoid compound greaterthan the saturation point of additional magnesium ions cannot beincorporated into the diamondoid-intercalated Mg/Al LDH.

The reaction temperature is chosen to provide sufficient thermodynamicenergy for the reaction of the magnesium salt and the diamondoidcompound to proceed within the reaction vessel and also to enablecrystallization of the Mg/Al-diamondoid-intercalated LDH. The reactiontemperature should be sufficiently high to enable the reaction toprogress but also be sufficiently low to avoid decomposition of theMg/Al-diamondoid-intercalated LDH or solvation of crystallites. In someembodiments, the reaction temperature may be from 100° C. to 200° C.,such as 100° C., 110° C., 120° C., 130° C., 140° C., 150° C., 160° C.,170° C., 180° C., 190° C., 200° C., or any other temperature between100° C. and 200° C. Though in some embodiments the reaction temperaturemay be from 100° C. to 200° C., the reaction temperature could be lessthan 100° C. or greater than 200° C. In other embodiments, the reactiontemperature may be from 100° C. to 150° C. or from 110° C. to 150° C. Inone example, where magnesium salt is Mg(OH)₂, the reaction temperaturemay be 150° C.±10° C.

The reaction time is chosen to provide sufficient time for crystalgrowth and for development of well-defined morphologies as theMg/Al-diamondoid-intercalated LDH is formed at the reaction temperature.In some embodiments, the reaction time may be longer than 12 h, such asfrom 12 h to 72 h, from 24 h to 72 h, from 12 h to 48 h, or from 24 h to48 h, for example. Though in some embodiments the reaction time may belonger than 12 h, it is contemplated that reaction times shorter than 12h may suffice, particularly when reaction temperatures greater than 150°C. are chosen.

Complete decomposition of the diamondoid-intercalated Mg/Al LDH mayinclude conversion of magnesium-hydroxide and aluminum-hydroxidefunctionalities to magnesium oxide and aluminum particles. Suitabledecomposition temperatures may be greater than 200° C., greater than300° C., greater than 400° C., or greater than 500° C., for example. Thedecomposition time may be chosen as any time sufficient to result incomplete decomposition of the Mg/Al-diamondoid-intercalated LDH at thechosen decomposition temperature. For example, the decomposition timemay be longer than 1 hour, such as 2 hours, 3 hours, 4 hours, or longerthan 5 hours. In example embodiments, Mg/Al-diamondoid-intercalated LDHsformed from Mg(OH)₂, Al₂(OH)₃, and ACA may decompose fully at adecomposition temperature of about 450° C. and a decomposition time ofat least 4 hours.

Nanocomposites formed by thermally decomposing theMg/Al-diamondoid-intercalated LDH may exhibit a variety of crystalmorphologies. The crystal morphologies may depend on variables, such asthe ratio of Mg²⁺ to Al³⁺ and the ratio of Al³⁺ to diamondoid compoundin the reaction mixture, the reaction time and temperature used to formthe Mg/Al-diamondoid-intercalated LDH, and the decomposition conditionsused to form the nanocomposite itself.

In some embodiments, the methods for preparing the layer metalnanocomposites include thermally decomposingMg/Al-diamondoid-intercalated LDHs prepared by reacting Mg(OH)₂,Al(OH)₃, and ACA. In other embodiments, the method for preparing layermetal nanocomposites includes thermally decomposingMg/Al-diamondoid-intercalated LDHs.

Nanocomposites formed from such Mg/Al-diamondoid-intercalated LDHs mayinclude magnesium oxide particles (MgO) of a particular shape ormorphology dispersed on a carbon support of a particular shape ormorphology. The metal-oxide particle may be spherical, rectangular,ribbon-like, or in the form of nanowires, nanorods, or nanowhiskers, forexample. The magnesium oxide particles may have particle sizes from 10nm to 20 nm, for example. Likewise, the carbon support may exhibit amorphology such as a sheet, a nanorod, a nanowire, or a nanowhisker.

The term “nanorod” means a nanoobject with two dimensions ranging from 1to 100 nm and the third dimension (length) being slightly greater. Theterm “nanowire” means a conducting anisotropic quasi-one-dimensionalstructure in which two external dimensions (such as width and thickness)are much smaller than the third dimension (length) are in the nanoscale.The term “nanowhisker” means a type of filamentary crystal (whisker)with cross sectional diameter ranging from 1 to 100 nm and length todiameter ratio greater than 100.

Without intent to be bound by theory, it is believed that upon thermaldecomposition of LDHs, interlayer anions escape as volatile gases andfurther heating results in the collapse of the layered structure. Thecollapse of the layered structure leads to the formation of mixed metaloxides that are agglomerated (larger particle size). In the preparationof nanocomposites according to embodiments, the interlayer anions do notdecompose, but rather polymerize to yield rod shaped adamantanemolecules. This enables the formation of layer by layer assemblies ofmetal oxides and carbon nanocomposites from the molecular level. Thus,preparation methods according to one or more embodiments not onlyprevent particle agglomeration, but also provide the support for themetal oxides.

The thermal decomposition of LDHs may progress through three observablesteps: (a) from room temperature to 100° C., adsorbed or physisorbedwater is removed; (b) in the temperature range of 100-220° C.intercalated water is removed; (c) at 220-400° C., intercalated anionsare removed and the mineral layers are dehydroxylated, leading to theformation of an amorphous mixed-metal oxide residue. Generally,intercalated anions completely decompose and escape as volatile gasesbefore the temperature reaches 450° C. Once the LDH is decomposed andwater and intercalated anions are removed, an amorphous residue of metaloxide is left behind. In some embodiments, the decomposition temperatureis from 220° C. to 450° C. In other embodiments the decompositiontemperature is from 300° C. to 450° C. In some embodiments, thermallydecomposing the diamondoid-intercalated Mg/Al LDH includes heating thediamondoid-intercalated Mg/Al LDH to the decomposition temperature at arate of 5° C. per minute.

The various embodiments described in this disclosure controldecomposition of the diamondoid-intercalated Mg/Al LDH. In addition tothe decomposition temperature and decomposition time, the atmosphericconditions control the decomposition of the diamondoid-intercalatedMg/Al LDH. When oxygen is not present during decomposition, thenanocomposite resulting from the decomposition is altered from that of ananocomposite decomposed in air. In one or more embodiments, the methodsfor preparing the layer metal nanocomposites include thermally treatingthe diamondoid-intercalated Mg/Al LDH the under a reducing atmosphere ata decomposition temperature for a decomposition time to form the layeredmetal nanocomposite.

In some embodiments, the thermal decomposition of thediamondoid-intercalated Mg/Al occurs under a reducing atmosphere. Thereducing atmosphere for the thermal decomposition of theMg/Al-diamondoid intercalated LDH prevents oxidation through a decreasein the amount of oxygen or other oxidizing gases or vapors in thevicinity of the reaction. The reducing atmosphere may contain activelyreducing gases such as hydrogen, carbon monoxide, or gases such ashydrogen sulfide that would be oxidized by any oxygen present. In someembodiments, the reducing atmosphere includes hydrogen gas (H₂).

In some embodiments, the decomposition of the diamondoid-intercalatedMg/Al LDH results in formation of a nanocomposite, in which magnesiumoxide particles may be uniformly dispersed over a surface of a carbonsupport. The carbon support may be derived from the adamantane moietiesof the diamondoid-intercalated Mg/Al LDH. The weight ratios of MgOparticles to carbon in the nanocomposite may vary, depending on theconditions by which the nanocomposite was prepared. In some embodiments,the nanocomposite may include from 50 wt. % to 90 wt. % MgO particlesand from 10 wt. % to 50 wt. % adamantane-derived carbon, based on thetotal weight of the nanocomposite. For example, the nanocomposite mayinclude from 70 wt. % to 80 wt. % MgO particles and from 20 wt. % to 30wt. % adamantane-derived carbon, based on the total weight of thenanocomposite.

Further embodiments of this specification are directed to catalystsystems. The catalyst systems may include (a) a diamondoid-intercalatedMg/Al LDH prepared according to any embodiment previously described; (b)a nanocomposite such as magnesium oxide particles supported on carbonprepared according to any embodiment previously described, such as bythermal decomposition of a diamondoid-intercalated Mg/Al LDH; or (c) anycatalytically active mixture of (a) and (b). The catalyst system may beused in olefin synthesis, transalkylation and dealkylation, photo orelectocatalytic water splitting.

Accordingly, further embodiments of this specification are directed tomethods for catalyzing a chemical reaction of at least one firstreactant with at least one second reactant. Such methods may includereacting the at least one first reactant and the at least one secondreactant in the presence of a catalyst system described previously. Theat least one first reactant and the at least one second reactant may beany chemical compounds, the chemical reaction of which is catalyticallyfacilitated, such as by being made thermodynamically possible or morefavorable or being kinetically influenced, by the presence of thediamondoid-intercalated Mg/Al LDH or the MgO nanocomposite separately orin combination.

EXAMPLES

The embodiments described in this specification will be furtherclarified by the following Examples. It should be understood that thefollowing Examples are not intended to limit the scope of thisdisclosure or its claims to any particular embodiment.

Example 1 Preparation of Diamondoid-Intercalated Mg/Al Layered DoubleHydroxides

Method A₁: Mg/Al molar ratio of 2:1 To prepare andiamondoid-intercalated Mg/Al layered double hydroxide materialaccording to an embodiment previously described, a 5% wt/wt solution ofMg(OH)₂ was prepared by dissolving 5 grams (g) of Mg(OH)₂ in 95 g ofde-ionized water. To the resultant solution, 3.36 g of Al(OH)₃ was addedin an amount sufficient to provide a Mg/Al molar ratio of 2:1. Then,9.31 g of adamantane carboxylic acid was added to the solution in anamount sufficient to provide an Al/adamantane molar ratio of 1:1 in theresultant reaction mixture. The pH of the reaction mixture was measuredand was found to be 9.5.

The reaction mixture then was stirred vigorously for 1 hour at roomtemperature. The stirred reaction mixture was transferred to aTeflon-lined autoclave and was heated at 150° C. for 24 hours (h) toform a the diamondoid-intercalated Al/Mg LDH as a precipitant. Thediamondoid-intercalated Al/Mg LDH was separated from the reactionmixture via gravity filtration. The pH of the filtrate was measure andwas found to be 8.6. The precipitant was washed thoroughly with wateruntil the filtrate had a pH of about 7. The precipitant was dried at 65°C.

Method A₂: Mg/Al Molar Ratio of 5:1

To prepare an diamondoid-intercalated Mg/Al layered double hydroxidematerial according to an embodiment previously described, a 5% wt/wtsolution of Mg(OH)₂ was prepared by dissolving 5 grams (g) of Mg(OH)₂ in95 g of de-ionized water. To the resultant solution, 1.34 g of Al(OH)₃was added in an amount sufficient to provide a Mg/Al molar ratio of 5:1.Then, 2.34 g of adamantane carboxylic acid, the diamondoid compound, wasadded to the solution in an amount sufficient to provide anAl/adamantane molar ratio of 1:1 in the resultant reaction mixture. ThepH of the reaction mixture was measured and was found to be 9.5.

The reaction mixture then was stirred vigorously for 1 hour at roomtemperature. The stirred reaction mixture was transferred to aTeflon-lined autoclave and was heated at 150° C. for 24 hours to form athe diamondoid-intercalated Al/Mg LDH as a precipitant. Thediamondoid-intercalated Al/Mg LDH was separated from the reactionmixture via gravity filtration. The pH of the filtrate was measure andwas found to be 8.6. The precipitant was washed thoroughly with wateruntil the filtrate had a pH of about 7. The precipitant was dried at 65°C.

The diamondoid-intercalated Mg/Al LDH resulting from the 2:1 molar ratioof Mg to Al (Method A₁) was characterized by IR spectroscopy, PXRD, EDX,proton and carbon solid state NMR.

The PXRD pattern of the as synthesized diamondoid-intercalated Mg/Al LDHis given in FIG. 1, and shows the basal reflection (001) at 20.84 Åcorresponds to a bilayer arrangement of adamantane ions in theinterlayer. The submultiples of (001) are seen at higher 20 values.Intercalation of adamantoic acid was further characterized with IRspectra (FIG. 2). The vibrations at 1517 cm⁻¹ and 1395 cm⁻¹ correspondto anti-symmetric and symmetric stretching vibrations of the COO⁻ group.The vibrations at 2901 cm⁻¹ and 2847 cm⁻¹ are for the C—H vibrations.The 4302 cm⁻¹ vibration arises from hydrogen bonding ofdiamondoid-intercalated Mg/Al LDH hydroxide groups with intercalatedwater molecules in the interlayer.

In FIG. 3, the ¹H spectrum of diamondoid-intercalated Mg/Al LDH showedfour sharp signals at lower parts per million (ppm) values owing to thehydrogen atoms present in the adamantane ring. The peak at 3.8 ppm and4.8 ppm are the result of hydrogen atoms of the intercalated water andmetal hydroxide respectively. In FIG. 4, the ¹³C NMR spectra ofdiamondoid-intercalated Mg/Al LDH showed four signals at 29.5 ppm, 37.3ppm, 40.6 ppm and 42.8 ppm indicate that there are four different carbonenvironments present in the adamantane molecule. The signal at 186.98ppm arises from the carbon of the carbonyl carbon of the carboxylategroup in the adamantane molecule. FIGS. 5A-5B are SEM micrographs ofMg/Al-adamantoate LDH, and depict the structural shape of thediamondoid-intercalated Mg/Al LDH. For example, SEM micrographs show thelayered particles have large surface area, but lacked thickness, therebyresulting in a high aspect ratio.

Example 2 Preparation of Mixed-Metal Oxide Nanocomposite

Mixed-metal oxides were prepared by thermally decomposing thediamondoid-intercalated Mg/Al LDH having a Mg/Al molar ratio of 1:1(Method A₁) of Example 1 at 450° C. for four hours under a reducingatmosphere of hydrogen gas. Upon thermal decomposition, LDHs yieldmixed-metal oxides. The diamondoid-intercalated Mg/Al LDH, yielded MgOand MgAl₂O₄ oxides.

The mixed-metal oxide nanocomposite resulting from thediamondoid-intercalated Mg/Al LDH resulting from the 2:1 molar ratio ofMg to Al (Method A₁) was characterized by IR spectroscopy, PXRD, EDX,proton, carbon, and aluminum solid state NMR, SEM, TEM, and HRTEM.

The layered metal nanocomposite was analyzed by PXRD (FIG. 6). Theresultant nanocomposite exhibited a series of basal reflections at 2θangles and the corresponding to d-spaces, 6.61° (13.36 Å) 7.98° (11.07Å), 9.23° (9.57 Å), 11.49° (7.7 Å), 14.0° (6.32 Å), 14.77° (6.0 Å),16.02° (5.52 Å), 17.09° (5.18 Å), 18.04° (4.91 Å), 42.71° (2.11 Å), and62.2° (1.29 Å). The reflections at 42.71° and 62.2° are typical ofmixed-metal oxides obtained by thermal decomposition of LDHs and areassigned to the MgO phase of the oxide residue. The multiple reflectionsat lower 20 values, specifically 6.61°, 7.98°, 9.23° and 11.49°, areunusual for mixed metal oxides of LDHs and in this case appear in thePXRD pattern as the result of the diamondoid compound in the resultantnanocomposite.

The nanocomposite was further characterized by IR spectroscopy, as shownin FIG. 7. The IR spectrum in FIG. 7 showed the symmetric andantisymmetric stretching vibrations of the COO— group at 1547 and 1417cm⁻¹. The IR spectrum showed the vibrations of the C—H groups at 2904and 2845 cm⁻¹. These vibration signals indicated the residual presenceof adamantane ion in the layered metal nanocomposite afterdecomposition. The vibration at 1217 cm⁻¹ was due to the C—O stretch ofthe adamantane ion. IR spectrum showed a weak broad vibration centeredaround 3434 cm⁻¹ due to O—H stretching of the hydroxyl ions.

Solid state NMR spectroscopy was used to gain the further structuralinformation of the resultant layered metal nanocomposite. In FIG. 8, the¹³C NMR spectrum of the nanocomposite showed signals at 29.5, 37.3,40.9, 42.3 ppm. These signals corresponded to the four unique carbonenvironments present in the adamantane ring. The signals in FIG. 8 aresimilar to the one observed in the Mg/Al-diamondoid intercalated LDH(FIG. 4), thus indicating that adamantane is present afterdecomposition. The signal at 186 ppm indicated the presence of thecarbon of the carbonyl in carboxylate ion; this signal was similar tothe Mg/Al-diamondoid intercalated LDH. However, a split was noticeablein all the signals including the one at 186 ppm. The splits, oroverlapping signals, indicated the formation of the second type ofadamantane ion, which was closely related to the parent adamantane ion.In addition to these signals, the resulting nanocomposite showed acarbon which resonates at 39.01 ppm. This signal did not appear in the¹³C NMR of the diamondoid-intercalated Mg/Al LDH (See FIG. 4). Thissignal could indicate the formation of new sp² carbon which acts as thelink between individual adamantane molecules.

A solid state Al NMR was performed to determine the chemical environmentof the aluminum in the nanocomposite. In FIG. 9, the ²⁷Al NMR spectrumof the layered metal nanocomposite showed two intense signals at 8.7 and69.2 ppm, which indicated that the Al³⁺ could be in either an octahedralenvironment or tetrahedral environment. The approximate ratio of Al³⁺present in octahedral sites to Al³⁺ present in tetrahedral site wasabout 3:1 determined by signal integration based on the intensity of thesignal in the NMR. In addition, the small hump at 36 ppm indicated thepresence of small amount of Al³⁺ in five coordinated geometry as well.

The surface morphology of the resultant nanocomposite was evaluated bySEM. The micrographs of FIGS. 10A-10D showed the formation of thelayered metal nanocomposite resulting from the decomposition ofMg/Al-diamondoid intercalated LDH. The images that are platelets aremixed metal oxides and the fibrous or rod shaped object are diamondoidsheets. The formation of the micron sized diamondoid sheets andsubmicron sized mixed metal oxides are much clearer in SEM ofnanocomposite in FIG. 11A-D. The layer-interlayer or layer by layernature of the as-prepared LDHs and its topotactic transformation intonanocomposite can be explained on the basis of SEM images (FIGS.10A-11D). Generally, LDHs upon thermal decomposition lose intercalatedwater and anions and hydroxyls ions in step wise manner; as a result,the layered structure of the LDHs would collapse to form mixed metaloxides. However, the controlled decomposition under reduced atmosphereof Mg/Al-diamondoid intercalated LDH led to the fusion of individualadamantane ions to grow as higher/longer diamondoids in the interlayergalleries as evidence by the rod or fibers shown in the SEM micrographs.Additionally, during decomposition, the metal hydroxides in the layershave lost their hydroxyl ions resulting in the formation of metal oxidesor the hydroxyl ions have been converted into the metal oxides. Due tothe layer-interlayer nature of the LDHs, the resulting metal oxides inthe layer are deposited on the higher/longer diamondoids formed in theinterlayer leading to the formation of layer-by-layer assembly ofnanocomposite at the molecular level.

HRTEM and TEM were used to understand the layer by layer assembly ofresultant nanocomposite. FIGS. 12A-13D show the several bright fieldimages. FIGS. 13A-13C are TEM images of the nanocomposite, and FIG. 13Dis an HRTEM image of the nanocomposite. The diamondoids aretubular/fibrous in nature, having length of several micron and diameterof around 2-5 nm. The long chain diamondoids are uniformly anchoredwithin the mixed metal oxides having uniform size of around 10 nm. Inaddition to the formation of the nanocomposite, the various embodimentsof the methods for preparing layered metal nanocomposites described amethod of controlling the size of the mixed metal oxide nanoparticles inthe composite.

The elemental analysis of the resulting layered metal nanocomposite wascarried out using EDX analysis by making use of the same sample gridwhich was used for the HRTEM analysis. The results of the elementalanalysis are shown in the FIG. 14. The approximate quantities ofdifferent elements in the nanocomposites were arrived at by using theintensity of the individual element peaks in the EDX spectra. The ratioof mixed metal oxide to carbon was found to be around 4. The atomicdistribution in the layered metal nanocomposite was characterized usingSTEM technique (FIG. 15A-15D). Mg, O and C have uniform atomicdistribution across the sample. The Al appeared in patches or wassegregated in pockets as expected for the mixed metal oxides of LDHs.

It should not be understood the various aspects of the methods forpreparing a layered metal nanocomposite, a layered metal nanocompositeprepared according to the same, and a catalyst system comprising thelayered metal nanocomposite prepared according to the same are describedand such aspects may be utilized in conjunction with various otheraspects.

In a first aspect, the disclosure provides a method for preparing alayered metal nanocomposite. The method comprises mixing a magnesiumsalt and an aluminum salt to form a Mg²⁺/Al³⁺ solution, in which theMg/Al has a molar ratio of between 0.5:1 to 6:1. Further, the methodincludes adding a diamondoid compound to the Mg²⁺/Al³⁺ solution to forma reactant mixture, in which the diamondoid compound has at least onecarboxylic acid moiety and heating the reactant mixture at a reactiontemperature for a reaction time to form a Mg/Al-diamondoid intercalatedlayered double hydroxide. Finally, the method includes thermallydecomposing the Mg/Al-diamondoid intercalated layered double hydroxideunder a reducing atmosphere for a decomposition time at a decompositiontemperature to form the layered metal nanocomposite.

In a second aspect, the disclosure provides the method of the firstaspect, in which the aluminum salt and the diamondoid compound are mixedin amounts that provide a ratio of Al³⁺ to diamondoid compound in thereactant mixture of from 0.5:1 to 2:1.

In a third aspect, the disclosure provides the method of the first orsecond aspects, in which the magnesium salt is Mg(OH)₂.

In a fourth aspect, the disclosure provides the method of any of thefirst through third aspects, in which the aluminum salt is Al(OH)₃.

In a fifth aspect, the disclosure provides the method of any of thefirst through fourth aspects, in which the diamondoid compound is1-adamantane carboxylic acid.

In a sixth aspect, the disclosure provides the method of any of thefirst through fifth aspects, in which the Mg²⁺/Al³⁺ solution is anaqueous solution.

In a seventh aspect, the disclosure provides the method of any of thefirst through sixth aspects, in which the reaction temperature is from100° C. to 180° C.

In an eighth aspect, the disclosure provides the method of any of thefirst through seventh aspects, in which the reaction temperature is from140° C. to 160° C.

In a ninth aspect, the disclosure provides the method of any of thefirst through eighth aspects, in which the decomposition temperature isfrom 220° C. to 450° C.

In a tenth aspect, the disclosure provides the method of any of thefirst through ninth aspects, in which the decomposition temperature isfrom 300° C. to 450° C.

In an eleventh aspect, the disclosure provides the method of any of thefirst through tenth aspects, in which the decomposition time is at least4 hours.

In a twelfth aspect, the disclosure provides the method of any of thefirst through eleventh aspects, in which the reducing atmospherecomprises hydrogen gas.

In a thirteenth aspect, the disclosure provides the method of any of thefirst through twelfth aspects, in which the layered metal nanocompositecomprises a layered morphology of a plurality of octahedral andtetrahedral environments.

In a fourteenth aspect, the disclosure provides a layered metalnanocomposite prepared according to the method of any of first throughthirteenth aspects.

In a fifteenth aspect, the disclosure provides a catalyst systemcomprising the layered metal nanocomposite of the fourteenth aspect.

In a sixteenth aspect, the disclosure provides the catalyst system ofthe fifteenth aspect, in which the layered metal nanocomposite comprisesa Powder X-Ray Diffraction (PXRD) pattern having multiple reflections atlower than 42.71 (2.11 Å) and 62.2° 20 (1.49 Å).

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments described inthis specification without departing from the spirit and scope of theclaimed subject matter. Thus it is intended that the specification coverthe modifications and variations of the various embodiments described inthis specification provided such modification and variations come withinthe scope of the appended claims and their equivalents.

1. A method for preparing a layered metal nanocomposite, the methodcomprising: mixing a magnesium salt and an aluminum salt to form aMg²⁺/Al³⁺ solution, in which the Mg/Al has a molar ratio of between0.5:1 to 6:1; adding a diamondoid compound to the Mg²⁺/Al³⁺ solution toform a reactant mixture, in which the diamondoid compound has at leastone carboxylic acid moiety; heating the reactant mixture at a reactiontemperature for a reaction time to form a Mg/Al-diamondoid intercalatedlayered double hydroxide; and thermally decomposing the Mg/Al-diamondoidintercalated layered double hydroxide under a reducing atmosphere for adecomposition time at a decomposition temperature to form the layeredmetal nanocomposite.
 2. The method of claim 1, in which the aluminumsalt and the diamondoid compound are mixed in amounts that provide aratio of Al³⁺ to diamondoid compound in the reactant mixture of from0.5:1 to 2:1.
 3. The method of claim 1, in which the magnesium salt isMg(OH)₂.
 4. The method of claim 1, in which the aluminum salt isAl(OH)₃.
 5. The method of claim 1, in which the magnesium salt isMg(OH)₂ and the aluminum salt is Al(OH)₃.
 6. The method of claim 1, inwhich the diamondoid compound is 1-adamantane carboxylic acid.
 7. Themethod of claim 1, in which the Mg²⁺/Al³⁺ solution is an aqueoussolution.
 8. The method of claim 1, in which the reaction temperature isfrom 100° C. to 180° C.
 9. The method of claim 1, in which the reactiontemperature is from 140° C. to 160° C.
 10. The method of claim 1, inwhich the decomposition temperature is from 220° C. to 450° C.
 11. Themethod of claim 1, in which the decomposition temperature is from 300°C. to 450° C.
 12. The method of claim 1, in which the decomposition timeis at least 4 hours.
 13. The method of claim 1, in which the reducingatmosphere comprises hydrogen gas.
 14. The method of claim 1, in whichthe layered metal nanocomposite comprises a layered morphology of aplurality of octahedral and tetrahedral environments.
 15. A layeredmetal nanocomposite prepared by: mixing a magnesium salt and an aluminumsalt to form a Mg²⁺/Al³⁺ solution, in which the Mg/Al has a molar ratioof between 0.5:1 to 6:1; adding a diamondoid compound to the Mg²⁺/Al³⁺solution to form a reactant mixture, in which the diamondoid compoundhas at least one carboxylic acid moiety; heating the reactant mixture ata reaction temperature for a reaction time to form a Mg/Al-diamondoidintercalated layered double hydroxide; and thermally decomposing theMg/Al-diamondoid intercalated layered double hydroxide under a reducingatmosphere for a decomposition time at a decomposition temperature toform the layered metal nanocomposite.
 16. A catalyst system comprising alayered metal nanocomposite prepared by: mixing a magnesium salt and analuminum salt to form a Mg²⁺/Al³⁺ solution, in which the Mg/Al has amolar ratio of between 0.5:1 to 6:1; adding a diamondoid compound to theMg²⁺/Al³⁺ solution to form a reactant mixture, in which the diamondoidcompound has at least one carboxylic acid moiety; heating the reactantmixture at a reaction temperature for a reaction time to form aMg/Al-diamondoid intercalated layered double hydroxide; and thermallydecomposing the Mg/Al-diamondoid intercalated layered double hydroxideunder a reducing atmosphere for a decomposition time at a decompositiontemperature to form the layered metal nanocomposite.
 17. The catalystsystem of claim 16, in which the layered metal nanocomposite comprises aPowder X-Ray Diffraction (PXRD) pattern having multiple reflections atlower than 42.71 (2.11 Å) and 62.2° 2θ (1.49 Å).